1. Field of the Invention
The invention generally relates to monitors used with computer systems and more particularly to a method and apparatus for blanking the display upon receipt of a key stroke until a password is entered.
2. Description of the Related Art
Personal computer systems are becoming more elaborate and are being used for more critical functions. For example, originally personal computers were only used for very simple word processing tasks and spreadsheet applications of relatively limited complexity. Today however, personal computers are being used to form elaborate network systems, both as the work stations and as the file servers and applications of great sensitivity or complexity are being executed. As the complexity of the various applications has gone up dramatically from a simple small spreadsheet application to extremely complex spreadsheets, so has the time required to perform the calculations. In many cases it is convenient for the user to do other tasks and leave his office, leaving the computer unattended. This unattended state is quite common in file server applications of the computer.
While the operator is away, the monitor continues to display. The monitor often does not, however, display anything of significance while the program is running. Because the operator is not watching the monitor, the display serves little purpose while the computer is performing its task and the operator is absent from the station. If the periods are extended, it is possible that the displayed image could be permanently imprinted or burned onto the monitor. Alternatively, sensitive data could be on the monitor or could be displayed while the operator is absent. It is undesirable to have this sensitive data always on display. For these reasons, it is desirable to blank the display from the monitor when it is not needed without turning the monitor off.
Screen blanking programs are well known in the art. Conventionally a screen blanking program blanks the screen after a set period of inactivity on the keyboard. Two interrupt routines, the timer interrupt and the keyboard interrupt, are changed to effect the blanking. In a computer's normal configuration, the timer interrupt is asserted periodically. The keyboard interrupt, on the other hand, is asserted every time a key is depressed or released on the keyboard. When either interrupt is asserted, the execution of the main code is suspended, and the processor looks in an interrupt vector table to find the starting address of the interrupt routine. The processor then branches to the appropriate address, runs the interrupt routine, and then returns to executing the main code.
The screen blanking program alters this progression. First, the program inserts a sequence in memory so that whenever the keyboard interrupt is asserted, a counter is reset. This means that every time a key is depressed or released on the keyboard, the counter is cleared. Next, a sequence is included so that whenever the timer interrupt is asserted, the counter is incremented by one. The counter value therefore reflects the amount of time that has passed since the last key strike on the keyboard. In the new timer sequence the counter value is compared to a previously set value that indicates the duration before the operator wants the screen to go blank. If the set value exceeds the counter value, the processor continues with the usual timer interrupt routine. If the counter value equals or exceeds the set value, however, the processor sends a command to blank the screen, and the display is blanked until the keyboard is used again. The next time a key on the keyboard is depressed, a keyboard interrupt is asserted, resetting the counter and bringing the display back onto the screen at the next timer interrupt.
Using a screen blanking program, the screen is automatically blanked while the keyboard (and presumably the computer) is idle. A disadvantage, however, is that anyone having access to the computer has access to the data on the screen by simply striking any key on the keyboard.
Another problem with the common screen blanking program is the delay before the screen is blanked. If the user wishes to blank his screen before leaving the station, he has no option but to wait until the delay has lapsed. Thus, it would be desirable to have a system in which the screen could be blanked at the user's command.
A program to blank the screen at the user's command would be executed by the system microprocessor. When the program was run, the screen would be blanked and the computer would scan the keyboard signals, looking for a password to reactivate the display. Because it is a separate program, however, the program could not be initiated while another program had control of the system. This problem could be avoided by using the system processor and a hot key scanning sequence. But certain operating systems block access to the keyboard interface. Consequently, signals between the keyboard and the main processor are blocked and the screen blanking process could not be initiated until the main program had run its course.
Therefore it is desirable to be able to blank the display on user command with out requiring the services of the system microprocessor.
In addition, security in the personal computer either locally or on a network is always of concern. For instance, if an administrator's station is connected into a network, the administrator will have various rights which other users are not allowed. If an ordinary user were to gain access to data in the network via the administrator's station, disastrous results could occur. Alternatively, if a program is in the midst of performing a very long recalculation and the operator wishes to leave for a period of time, another party could come in and see the computer doing apparently nothing and hit a key on the keyboard, interrupting the program in midstream. To prevent these mishaps, a user may lock his keyboard using a keyboard password lock as described in patent application Ser. No. 525,490, entitled "Keyboard Password Lock", filed on May 18, 1990, which is hereby incorporated by reference. The password lock prevents access to the computer through the keyboard without a password, yet it does not interrupt the main processor functions as it performs its task. While the keyboard is locked, the display is usually unnecessary, and is again needlessly wasting its operational life. For these reasons it is desirable to be able to blank the screen so that monitor life is not reduced and sensitive data not displayed and to lock access so that information cannot be obtained without clearance.